A radio-altimeter is an avionics system whose function is to determine the height (called “radio-altimetric height”) of the aircraft above the ground, that is to say the vertical distance between the ground being flown over and the aircraft. This system is based on the measurement of the time that elapses between the emission of a frequency-modulated carrier and the reception of this carrier after reflection by the ground. The radio-altimeter is used, notably, for precision approaches on a transport airplane.
To satisfy the requirements of the embedded systems which use the radio-altimetric data (namely the “radio-altimetric heights”) determined by the radio-altimeter, and notably those of an automatic piloting system, the radio-altimeter has to provide an item of information on height from 0 feet to approximately 5000 feet corresponding to its operational range. Beyond this height, the radio-altimeter no longer receives the signal reflected by the ground and therefore transmits a fixed value to these user systems, accompanied by an information item indicating that the radio-altimetric datum can no longer be determined.
During flight phases which are situated above this operational range (that is to say above 5000 feet), the radio-altimeter can provide a value which corresponds to any object which would be located within the radiation pattern of its antennas, such as, for example, when flying over another aircraft that is cruising where the spacing between the aircraft is generally around 1000 feet.
The possibility of detection of an object flown over by the radio-altimeter at any moment outside of its operational range requires the user systems to implement coherence logics to avoid inappropriate behavior. These logics tend to increase the complexity of the implementation of these user systems.